1. Field of Invention
This invention relates to an electronic overvoltage protection device with a fast-operation feature, intended for a power transistor having a control terminal of the MOS type. More particularly, the invention concerns a device as above, intended for a power transistor having at least one control terminal of the MOS type, which device is of the type comprising a Zener diode associated with the power transistor and jointly integrated in a substrate.
2. Discussion of the Related Art
As those skilled in the art know well, a power transistor, such as a Power-MOS or IGBT transistor, must be protected during operation against overvoltages. In particular, such a transistor is to be given protection from any overvoltages presented to its drain terminal, or collector terminal for an IGBT transistor. An overvoltage occurring across the transistor, and specifically the presentation of a high potential to either the drain or the collector terminal thereof, can have different origins. For example, an overvoltage can be caused by an emf induced by current variations in the loads being driven from the power transistor, or by a voltage peak on the power supply distribution line.
In any case, upon the occurrence of an overvoltage, a large current may flow through the transistor which can force it to operate outside its safe range, and in some cases may result in the transistor being destroyed. Some proposals for providing power transistors with protection from overvoltages are known in the prior art.
A first approach is illustrated in FIG. 1, which shows at least one high-voltage Zener diode Z placed between the collector or drain terminal C and the control or gate terminal G of the power transistor. When the voltage at the collector reaches the threshold voltage level of the Zener diode, the resulting current flow through the diode charges the gate G of the transistor, which will be turned back on and clamp the voltage on the collector terminal C to a value equal to the combined values of the threshold voltage of the Zener diode Z, Vth and the voltage drop at the gate terminal G of the transistor, Vg.
During the time when the voltage remains clamped to the value Vc, the current flowing through the Zener diode obeys the following relationship: EQU Iz=Vg/Rd=(Vth+gm*Id)/R.sub.goff ;
where Vg is the voltage at the gate terminal, R.sub.goff is the drive resistance connected to the drain terminal which will only be at work with the Zener diode in operation, gm is the transconductance of the power transistor, and Id is the drain current.
The current Iz through the Zener diode can attain very high values, especially when the drive resistance is selected low in favor of a high transistor switching speed. The protection device including the Zener diode is usually integrated in a semiconductor, to the same electronic circuit with which the power transistor is incorporated.
The prior art proposes a first way of integrating the Zener diode together with the power transistor. FIG. 2 shows schematically a vertical cross-section, drawn to an enlarged scale, of the structure of a high-voltage Zener diode integrated to an isolation well 2 which has been doped P-. Located over the well 2 is an N- doped buried region 3 which forms a PN junction with the well 2. The well 2 and region 3 are contacted by respective anode and cathode terminations.
This type of structure has, however, certain disadvantages. The power transistor structure with which this Zener diode protector is to be associated must be modified, which involves increased fabrication complexity and higher cost; the aforementioned PN junction involves the creation of a parasitic bipolar transistor of the npn type which may be triggered on during fast transients; and a Zener diode so constructed exhibits a large thermal drift about the value of the threshold voltage.
The prior art also proposes a second approach to providing a protector, which incorporates at least one Zener diode. FIG. 3 shows schematically a vertical cross-section, also to an enlarged scale, through the structure of a high-voltage Zener diode formed of a chain 5 of polycrystalline silicon diodes. This chain of diodes comprises basically a series of PN junctions of polycrystalline silicon deposited over an oxide layer 4 which is covering a semiconductor substrate 6. Respective anode and cathode terminations are provided in contact with the opposed ends of the diode chain 5.
This second prior art approach has certain advantages. Specifically, there is no need for additional processing steps, there is an absence of parasitic components and it is stable through temperature changes.
However, a Zener diode formed of a chain of polysilicon diodes has a series resistance of relatively high value. If the resistance between the gate and source terminals of the MOS power transistor is small, the Zener voltage combines with a large voltage drop across the series resistance. This causes a rise in the clamping voltage, which may then become as high as the breakdown voltage of the power transistor.
With the resistance between the gate and source terminals corresponding to the drive resistance on which the switching time of the power transistor is dependent, it may be appreciated that the polycrystalline silicon Zener diode does not suit fast-switching circuits.
In summary, the prior art approaches have been unable to provide a voltage-clamping protection device which can be effective with high switching speeds without introducing additional fabrication process steps and parasitic elements in the power transistor.
The underlying technical problem addressed by the present invention is the problem of providing a protection device, particularly for power transistors, which has constructional and functional features to ensure high switching speeds, but involves no additional process steps for its manufacture and introduces no parasitic elements in the power transistor.